1. Field of the Invention
This invention generally relates to circuit breakers and, more particularly, to a circuit breaker of the molded case type that has an improved contact arm assembly, toggle link means and an arc shield component that improve the performance and reliability of the circuit breaker and reduce its cost.
2. Description of the Prior Art
Circuit breakers of the molded case variety are well known in the art and generally comprise a pair of separable contacts that are opened and closed by an overcenter toggle assembly and a spring-powered operating mechanism that is releasably latched by a current-responsive trip means which is magnetically and/or thermally actuated. Such circuit breakers are disclosed and described in detail in U.S. Pat. Nos. 3,480,900 (Gelzheiser et al.); 3,492,614 (DeAngelo et al.); 3,559,156 (Coley); and 4,163,881 (Borona et al.).
While circuit breakers having the structural and design features embodied in the various units disclosed in the aforementioned patents provided the desired protection against current overloads for the electrical circuits in which they were employed, they had certain features which inherently detracted from the performance of the breakers and increased their manufacturing cost. In accordance with the prior-art breaker designs, the movable contact arm was pivotally mounted between a pair of support plates by a pivot shaft arrangement which permitted the contact arm to shift position laterally along the shaft when the contact arm swung toward and away from the stationary contact during circuit breaker operation. This shifting was limited to some degree by the coiled segments of the biasing spring which encircled the pivot shaft on either side of the movable contact arm.
The resulting variation in the lateral position of the contact arm on the pivot shaft produced corresponding variations in the spacing or gap between the contact at the free end of the arm and the arc shield as the contact swung through the shield during the contact-opening stroke of the circuit breaker. The non-uniform spacing between the movable contact and arc shield delayed the rapid quenching of the arc since the arc would inherently be concentrated in the part of the gap where the movable contact and arc shield were in closest proximity.
In accordance with the prior-art breaker designs, the movable contact arm was also coupled to the cradle link of the operating mechanism by a toggle assembly that included two pairs of toggle links which were pivotally coupled to one another by pins having protruding end portions that were fitted with force-fitted lock washers (known in the art as "Tinnerman" washers). Spacer washers were also placed between the pair of handle links and cradle links at the point at which they coupled to the lower and upper pairs of toggle links. The assembly of the toggle links, spacer washers, and the various pivot pins and lock washers was accordingly very difficult to perform in an efficient manner on a mass production basis.
Another disadvantageous feature of the prior-art circuit breakers was the utilization of an arc shield that had an angular-shaped grid portion which inherently reduced the arc gap.